1. Field of the Invention
The present invention relates generally to fluid ejection modules and, more particularly, to a heater chip for an inkjet printhead chip module having a silicon die bonded on a silicon substrate and methods of fabricating the heater chip.
2. Description of the Related Art
A thermal inkjet printhead is used in a variety of products, such as printers, copiers, faxes and the like, to form and propel drops of ink onto a sheet of print medium, such as paper. In general, an image is produced by the ink drops being emitted from the printhead at precise moments such that they impact the print medium at a desired location. The printhead is supported by a movable carriage within the product and is caused to reciprocate relative to the advancing print medium. It emits the ink drops at times pursuant to commands of a microprocessor or other controller. The timing of the ink drop emissions corresponds to a pattern of pixels of the image being printed.
One thermal inkjet printhead of a conventional design used in such products is illustrated diagrammatically (not to scale) in FIG. 1, with an enlarged fragment of the design illustrated diagrammatically in FIG. 2. The printhead 10 has a chip module 14 composed of a heater chip 20, a nozzle plate 22 attached to or integrated with the heater chip 20, and an input/output connector (not shown), such as a tape automated bond (tab) circuit, for electrically connecting the heater chip 20 to the printer during use. The heater chip 20 has a single silicon substrate 24 seated within and adhesively bonded at 28 to the recessed mouth portion 16 of the bottle 18 on the base plate 12 thereof. The heater chip 20 also has electrical circuitry which includes a plurality of electrical logic components (not shown) and resistors or heater elements 30, as seen in FIG. 2, built on the silicon substrate 24. Each heater chip 20 of a given printhead chip module 14 is fabricated using conventional microelectronic manufacturing techniques and, more particularly, as part of a multiplicity of heater chips processed on a single silicon wafer following conventional semiconductor processing steps. Thus, it will be understood that the heater elements 30 are an integral part of the upper portion of the silicon substrate 24, i.e. they are part of the electrical device, of the heater chip 20, as seen in FIG. 2.
The conventional design, illustrated in FIGS. 1 and 2 and similarly disclosed in U.S. Pat. No. 6,402,301, which is hereby incorporated by reference in its entirety, is effective for today's print quality demands; however, it is apparent that the system described above has print quality limitations, invites long-term corrosion issues, and is less than simple to manufacture. The shortcomings of the conventional design insofar as they affect print quality relate primarily to ink drop misdirection and ink drop size.
Regarding ink drop misdirection, anomalies which cause it are ones that adversely affect nozzle planarity, bore angle of nozzle sidewalls, ink drop velocity, heater/nozzle plate alignment, and dimensional relationships between components. Nozzle planarity determines the direction the ink is dispensed (i.e. drop misdirection). If the nozzle plate is warped or bowed, the desired direction of the ink-jetting is compromised. Nozzle planarity is affected by a mismatch of the coefficient of thermal expansion (CT) between the base plate of the ink bottle and the silicon substrate of the heater chip, between components of the chip module itself, and between the die bond adhesive layer, encapsulant material and the heater chip. Further affecting nozzle planarity are planarity of the base plate in the bottle, non-uniform deposition of either the FF material or a photo-imagable nozzle plate layer impacting formation of the nozzle holes, and sagging of the nozzle plate into the via due to lack of support over the via causing misdirection of nozzle holes.
Bore angle, the angle of the sidewalls of the nozzle holes through the nozzle plate, is ideal when it is reentrant, i.e. a narrow opening at the surface of the nozzle plate over a wider opening at the base. Current technologies are severely limited in their ability to repeatably produce this feature. Repeatable bore angle is fundamental to limiting ink jetting misdirection and drop velocity Ink drop velocity is adversely affected by distortion of the dimensions of the heater elements and the size of the ink ejection chambers. Heater size is limited by the heater chip surface space.
The heater chip/nozzle plate alignment in the chip module is affected by how well the nozzle plate is aligned with the heater elements, which is proving to be very difficult to accomplish. Also, expansion and contraction of the nozzle plate resulting from other wet and dry processing will affect this alignment. Anomalies affecting dimensional relationships, such as X-spacing, Y-spacing, Z-height, skew and tilt, are the CTE mismatch of the base plate and substrate, the flatness of the base plate, the repeatability and precision of tab bond and tab attach, and the CTE mismatches between heater chip/die bond adhesive material.
Thus, there is a need for an innovation to overcome the above-mentioned shortcomings of the conventional design of the printhead chip module so as to reduce the deleterious effects of the printhead chip module design on print quality.